Expose GLES 3.0 feature of half_float vertex on GLES 2.0 as an extension.

Bug: angleproject:3191
Test: dEQP-GLES2.capability.extensions.vertex_data_formats.GL_OES_vertex_half_float
Change-Id: Ia093b66f9c32c81946b2cbc15ff227baea4bfb02
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1728749Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Mohan Maiya <m.maiya@samsung.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>